This invention relates generally to irrigation equipment of a type designed for delivering irrigation water to crops and the like at a relatively slow substantially drip flow rate. More particularly, this invention relates to improved and economical continuous tube emitters, wherein the tube has a plurality of spaced emitters through which the flow rate of irrigation water is controlled and regulated in response to water supply pressure.
A variety of so-called continuous tube emitters for irrigation purposes are generally known in the art. Such continuous tube emitters typically comprise an elongated tube of rigid or flexible construction having a plurality of outlet openings formed along its length and through which irrigation water is discharged at a relatively slow, substantially drip flow rate, such as a flow rate less than about one gallon per hour. With such devices, it is intended that irrigation water be discharged at selected points along the tube for delivery to adjacent crops and the like without significant water waste from evaporation and further without significant soil erosion.
Previously available continuous tube emitters have utilized many different structural techniques intended to leak water slowly through outlet openings at spaced intervals along the length of the tube. For example, continuous tube emitters have been suggested with fibrous or other porous substances occluding the outlet openings to permit slow water leakage therethrough, such as those devices depicted in U.S. Pat. Nos. 3,777,987 and 2,799,422. Other continuous tube emitter designs have proposed concentric or multiple tube constructions wherein irrigation water is leaked through a series of small pressure- and flow-reducing orifices. See, for example, the emitter designs shown and described in U.S. Pat. Nos. Re. 28,095; 3,361,359; 3,672,571; 3,903,929; 4,534,515; and 4,626,130. Still other tube emitter designs have proposed relatively complex and elongated or labyrinth flow path configurations for reducing the flow rate and pressure of water discharged through outlet openings, such as those devices depicted in U.S. Pat. No. 4,002,684; 4,077,570; 4,077,571; and 4,763,842. However, in these exemplary continuous tube emitter designs, the requisite low water flow rates have required outlet openings or leakage paths of fixed, small cross-sectional area which are highly susceptible to clogging by dirt or other particulate matter commonly present in most water supply systems. Additionally, such designs have not heretofore been capable of reliably and consistently producing a substantially constant flow rate over the full range of working pressures normally encountered in use.
In U.S. Pat. No. 4,807,668 there is disclosed a continuous tube emitter formed by folding a plastic web lengthwise with the edges overlapped and heat sealed together, and which has a preformed groove along one edge to define an elongated secondary conduit of small cross-section within the seam. Small openings are formed at spaced intervals along the seam of the secondary conduit which function as outlets from the tube. With this construction, the inner wall portion of the overlapped web forms a dividing wall between the main conduit and the secondary conduit, and which is said to be capable of producing a throttling effect by deflecting into the groove in response to increased water pressure within the main conduit so as to reduce the size of the secondary passageway and thus regulate the outlet flow. While several attempts to produce flow control devices have employed this approach, none has been capable of precisely and consistently producing a substantially uniform flow rate over the full range of working pressures normally encountered. This is believed to be due to the inability of the inner wall portion to effectively constrict the size of the secondary passageway in response to pressure increases since that wall is subjected to a tensile force created by the internal water pressure within the main conduit and which prevents the wall from appreciably deforming in a controlled manner into the groove. That is, the internal pressure within the main conduit attempts to inflate the tube, thereby placing the tube wall under a tensile load. As the internal water pressure increases, the tensile force on the tube wall also increases, this tensile force actually reducing the ability of the wall to controllably deform into the groove.
A further continuous tube emitter which is intended to overcome many of the foregoing problems is that disclosed in U.S. Pat. No. 4,726,520, assigned to the assignee of the present application. That continuous tube emitter design provides a flexible plastic tube formed from an elongated thin film web having one or more relatively thicker valve members on one side thereof defining a plurality of valve faces, wherein each valve face includes at least one shallow drip emission groove leading into a valve reservoir of wider cross section and communicating respectively with outlet openings through the web. One longitudinal margin of the web is trimmed to form laterally projecting flaps at longitudinal positions generally corresponding with the valve faces. The web is then rolled upon itself about a longitudinal axis and longitudinally seamed to form the continuous tube emitter with the flaps each internally overlying the valve reservoir and a portion of the drip emission groove of a respective valve face to define one of the flow control units. In use, water pressure within the tube forces the flaps into engagement with the aligned valve faces to restrict and control the rate of water flow through the outlet openings. While the foregoing construction provided an improved and more uniform flow rate over normal working pressure range, it was found that the flap-type construction was difficult to consistently manufacture, and that the level of flow was not always predictable.
Typically, continuous tube drip systems are designed to be operational over a working range of pressures, normally between approximately 6 and 10 pounds per square inch. In the event the continuous tube drip system is employed in hilly terrain, the effective working pressure range may be higher for those portions of the tube lying in valleys, and lower for those portions of the tube overlying the crest of a hill, typically by as much as plus or minus two pounds per square inch. Accordingly, it is highly desirable to be able to compensate for such pressure variations to insure that a substantially uniform flow rate from each emitter along the length of the tube is obtained even though substantial pressure variations are present.
The response of any give emitter in a continuous tube emitter system can be characterized by the formula Q=c P.sup.x where: Q=the emission flow rate at the outlet port of the emitter; P=the pressure inside the emitter tube; c =a constant coefficient which can be empirically determined for each type emitter; and x=an emission rate exponent. From this equation, it can be seen that if the emission exponent equals one, then the emission rate is directly proportional to the pressure inside the emitter tube. That is, if the pressure within the tube doubles, the flow rate from the emitter will double. For an emission rate exponent equal to zero, on the other hand, the emission rate is constant regardless of pressure inside the emitter tube. Thus, for any emission rate exponents value greater than zero but less than one, the emitter will have a pressure compensating characteristic. Since it is always desirable to have a substantially constant flow regardless of the water pressure within the emitter tube, it is therefore desirable to attempt to obtain an emission rate exponent as close to zero as possible.
As will become more apparent hereinafter, the present invention provides a drip irrigation tube having discrete emitter elements which are capable of producing a substantially constant outlet flow rate over the entire range of working pressures typically encountered in use, and whose emission rate exponent is extremely close to zero.